1. Field of the Invention
The invention relates to a radio frequency spectrum analyzer and, in particular, to an acousto-optic heterodyne spectrum analyzer. More specifically, the invention relates to a spectrum analyzer of this type having improved inherent optical stability.
2. Prior Art
Instantaneous spectral analysis of an unknown radio frequency (rf) signal into a large number of discrete output channels, by optical diffraction from a corresponding acoustic wave in a Bragg cell, is a known application of acousto-optics. To increase the dynamic range of optical detection at the Fourier-transform plane of the Bragg cell and to retrieve the phase information in the transform, heterodyne or interferometric techniques have been used, where generally a second Bragg cell driven by a broadband rf reference introduces a set of optical local oscillator beams that mix with the unknown signal beam to produce channel outputs that are amplitude and phase modulations of an IF carrier. The IF carrier frequency is desired to be the same for all channels and is generally low compared to the unknown signal frequency.
However, the use of two Bragg cells in this way introduces severe implementation difficulties, especially when the system is required to operate in a noisy environment. The Bragg cells (BC1, BC2) must be placed into different arms of an interferometer, such as a Mach-Zehnder (M-Z) interferometer shown in FIG. 1. The two arms are adjusted so that light from a source S, diffracted by a first beam splitter (BS1) into two beams (1 and 2), corresponding to the unknown signal and to the local oscillator reference signal, are combined at a second beam splitter (BS2) and propagated together to a detector (D) where they generate the heterodyne signal. Both arms must be held stable with respect to each other to maintain constant heterodyne efficiency and to minimize phase changes that can corrupt the accuracy of the phase information. For example, there may be slowly changing alignment of the optical components due to differential thermal effects, plus rapid alignment changes due to vibration of components. In the Mach-Zehnder interferometer, these result in time-dependent misalignment between the output beams and unequal differential path length changes which modulate both the phase and amplitude of the heterodyne signal. In applications where valuable information is contained in the phase and where good short term and long term phase stability are both needed, the necessary stability is difficult to achieve in independent path arrangements such as the Mach-Zehnder.
To obtain a steady noise-free heterodyne signal, the two diffracted beams must remain parallel. However, in the arrangement of FIG. 1, if there is an unwanted angular disturbance of the Bragg cells (BC1, BC2) or the mirrors (M1, M2) in any direction, the angle between the output beams is changed by twice the angular disturbance. Since the two paths are independent, a disturbance in one path can change the path length of that path while the other path length remains unchanged. This differential path length change can cause an advance or retardation of the phase of the heterodyne signal from the detector. This phase noise or "jitter" can mask the phase information on the rf signal.
Also, in the arrangement of FIG. 1, in order to obtain a non-zero frequency in the heterodyne signal, the two diffracted beams must be carefully misaligned by a very small angle corresponding to the frequency difference between the rf signal and reference frequencies. The Mach-Zehnder type interferometer is difficult to align because of the many independent degrees of freedom which must be adjusted, and it is likewise difficult to obtain a small angle offset.
By improving the optical stability of the spectrum analyzer, a number of advantages are possible. The noise modulation on the amplitude and phase of the heterodyne signal due to unwanted ambient vibrational disturbances is reduced. Improved alignment stability of the output beams allows greater spectral resolution and hence a larger effective time-bandwidth product, i.e., a larger number of heterodyne channels, and reduced amplitude modulation allows the theoretical dynamic range to be approached more closely. Finally, increasing the inherent stability of the optical system reduces the need to take extreme measures to obtain stability by other means, such as by a massive, ultrarigid construction.
One way to minimize vibrational and thermal instability in interferometric systems is to use a suitable common-path arrangement in which some or all of the misalignments cancel, due to symmetry and the fact that both beams are made to follow the same path. The triangular, common-path interferometer (TCPI) is one such arrangement which also provides a smaller number of mechanical components.
However, the reduced number of mechanical degrees of freedom in the TCPI which leads to improved stability, makes the two output beams always parallel to each other, and thus cannot be misaligned by ordinary means to provide for the desired fixed intermediate frequency (IF) offset between the signal and reference frequencies. Also, since the TCPI requires both beams to pass through both Bragg cells, this results in reduction of the beam energy, or beam depletion. Beam depletion puts an unwanted "complementary" version of the reference channel signal into the signal channel and vice-versa, delayed in time by the increased path to the detector. This unwanted signal, or "crosstalk", interferes with the desired heterodyne signal and wastes optical power, so it must be minimized.
Implementation of an interferometric spectrum analyzer is most difficult where the number of output frequency channels is large. Acousto-optic technology presently allows the decomposition of a frequency band into more than 1000 narrow-frequency channels that are detected by the pixel elements of a photodetector array 33. By using currently available Bragg cells, 40-us time apertures can be achieved with 50 MHz bandwidth, giving a time-bandwidth product, or number of channels, of 2000 with high diffraction efficiency, on the order of 50% per rf watt and greater. These are large aperture, on-axis cut tellurium dioxide (TeO.sub.2), acoustic shear wave Bragg cells.
Examples of pertinent prior art are described in U.S. Pat. Nos. 2,764,055, 3,473,031, 4,636,718 and 4,725,774. Davis et al. (4,725,774) relates to an interferometric acousto-optic spectrum analyzer which avoids some of the problems associated with the Mach-Zehnder arrangement by integrating the optics on a substrate. Geometrically, this invention uses the unstable Mach-Zehnder arrangement, where reflectors and beam splitters are replaced by Bragg diffraction cells, but does not suffer as much mechanical instability as with discrete optics because integrated-optic techniques are used.
White (3,473,031) relates to a triangular common-path laser transmitter for generation of simultaneous frequency modulated and unmodulated beams. The clockwise and counterclockwise laser beams are polarization separated over a portion of the path between two Faraday rotators. A Kerr cell or other light modulator between the rotators phase modulates the beams differently. Clemens et al. (2,764,055) is similar, and relates to the precise rotation of the plane of polarization of light beams traveling in opposite directions using Faraday rotators instead of Kerr cells for a more stable rotation angle, illustrated by a TCPI arrangement which is not essential to the invention. However, stable rf or microwave spectrum analysis is not possible with either of these devices.
Labrum et al. (4,636,718) relates to an acousto-optic spectrum analyzer of the standard non-heterodyne type using a single Bragg cell. Electronic circuitry is used to expand the frequency resolution.